Troponin together with tropomyosin, regulate the binding of myosin to actin. Troponin is a trimeric protein composed of Troponin subunits I, C and T. Troponin C binds calcium ions, Troponin T binds to tropomyosin and troponin I binds to actin. This calcium-dependent interaction serves to position the tropomyosin along the actin filament during muscle contraction. Troponins are used as diagnostic biomarkers for cardiac injury.
Detection of skeletal muscle injury is hampered by a lack of commercially available assays for serum markers specific for skeletal muscle. Although CK is the most common serum marker for skeletal muscle injury, it is not ideal for several reasons, including lack of tissue specificity, inability to reveal damage to specific skeletal fiber types (fast or slow), and inappropriately low values when glutathione concentrations are decreased because of liver or multiple-organ failure. Skeletal muscle troponin-I is the marker of choice for detection of muscle injury because unlike myoglobin and heart-type FABP it is expressed exclusively in skeletal muscle. Myoglobin and heart-type FABP are useful markers of skeletal muscle injury in the absence of cardiac damage. Skeletal troponin I (sTnI), with its two distinct isoforms [fast (fsTnI) and slow (ssTni)], like cTnI and cTnT, may have an advantage over conventional markers for detecting skeletal muscle injury. Moreover, because sTnI exists in 2 isoforms, slow (ssTni) and fast (fsTnI), corresponding to slow- and fast-twitch muscles, respectively, it could provide insight into differential injury/recovery of specific fiber types.